1. Field of the Invention
The present invention relates to a motor for use in a floppy disk drive or the like, and more particularly to an index detecting portion of the motor.
2. Description of the Related Art
In a motor for use in a floppy disk drive or the like, a reference rotational position of a rotor is detected, that is, an index position is detected.
Referring to FIGS. 20 and 21, there is shown a conventional motor including a rotor yoke 1, an index detecting magnet 2 mounted on the outer circumference of the rotor yoke 1 and adapted to rotate with the rotor yoke 1, and a magnetic sensor 3 such as a Hall element opposed to the outer circumference of the rotor yoke 1 for detecting magnetic flux generating from the index detecting magnet 2. A detection signal from the magnetic sensor 3 is input to a processing circuit (detecting circuit) (not shown), which in turn generates one index signal per revolution of the rotor. Reference numeral 4 in FIG. 20 denotes a stator provided with a coil (not shown). In FIG. 21, the stator 4 is not shown.
In the motor shown in FIGS. 20 and 21, the index detecting magnet 2 is necessary and the work for mounting the magnet 2 to the rotor yoke 1 is also necessary. Accordingly, there is a problem in size reduction and cost reduction of the motor and a device using the motor. To solve this problem, there has recently been proposed an improved motor having such a structure that a recess is formed on the side surface of a rotor yoke to expose a part of a magnet inside the rotor yoke, whereby magnetic flux from the magnet partially exposed to the recess is detected by a magnetic sensor to thereby perform index detection.
Referring to FIGS. 22 and 23, there is shown a rotor 8 of such an improved motor. The rotor 8 is provided with a rotor yoke 5 and a rotating magnet 6 having N poles and S poles axially magnetized at equal intervals. The rotor yoke 5 is formed with a rectangular recess 7 to which a part of the outer circumference of the rotating magnet 6 is exposed. As shown in FIG. 22, the poles of the rotating magnet 6 are such that when the upper side (front side) of the magnet 6 has an N pole, for example, the lower side of the magnet 6 (the side opposed to a stator not shown) has an S pole. In FIGS. 22 and 23, the poles of the magnet on the lower side are denoted by reference numerals 6a to 6d. The N pole 6b and the S pole 6c are exposed to the recess 7 partially to the same degree. The magnetic flux generating from the rotating magnet 6 partially exposed to the recess 7 is detected by a magnetic sensor (e.g., Hall element) provided in opposition to the outer circumference of the rotor yoke 5. The magnetic sensor detects a radial component of the magnetic flux generating from the rotating magnet 6. FIG. 24 shows a detection characteristic of the magnetic sensor (a change in magnetic flux density with time) when the rotor 8 is rotated in the direction depicted by an arrow A in FIG. 23. In FIG. 24, the upper side (plus side) represents magnetic flux directed from the center of the magnet 6 to the outside thereof, and the lower side (minus side) represents magnetic flux directed from the outside of the magnet 6 to the center thereof. Reference characters a to e in FIG. 24 denote magnetic flux densities at different positions shown by the corresponding reference characters in FIG. 23.
As apparent from FIG. 24, at the point b where the N pole 6b is opposed to the front of the magnetic sensor, a large output (magnetic flux density) on the plus side is obtained. At the boundary point (point c) between the N pole 6b and the S pole 6c, a zero output is obtained. When the rotor 8 is further rotated and the S pole 6c comes to a position opposed to the front of the magnetic sensor, a large output on the minus side is obtained (at the point d). Then, a processing circuit (not shown) connected to the magnetic sensor specifies the point c where the output is zero as an index position, thus detecting a rotational position of the rotor. That is, the point c where the output becomes zero after it exceeds a threshold Th is specified as the index position. The reason why the threshold Th is set is that while the magnetic sensor also detects the magnetic flux generating from a portion of the rotor yoke 5 where the recess 7 is not formed and generates a noise signal, this noise signal is intended to be distinguished from a signal based on the magnet exposed to the recess 7. The threshold Th can be set as a hysteresis width of a comparator in the processing circuit (not shown), that is, as a difference between an input voltage of the comparator where an output from the comparator changes from H level (e.g., 5 V) to L level (e.g., 0 V) and an input voltage where the output changes from L level to H level. The hysteresis width (threshold Th) is decided from a resistance of a resistor connected to the comparator.
In the motor shown in FIGS. 22 and 23, the S pole 6a adjacent to the N pole 6b on the upstream side in terms of rotation of the rotor 8 is not exposed to the recess 7. Accordingly, the magnetic flux directed from the N pole 6b to the S pole 6a is also leaked and it is also detected by the magnetic sensor as shown by the point a in FIG. 24. Then, a signal (overshoot) on the minus side indicating the magnetic flux directed to the center of the rotating magnet 6 is output at the point a. Similarly, the magnetic flux directed from the N pole 6d not exposed to the recess 7 to the S pole 6c exposed to the recess 7 is also detected by the magnetic sensor, and a signal (overshoot) on the plus side is output at the point e from the magnetic sensor. The signal levels of these overshoots are greater than the signal level of noise detected from the portion of the rotor yoke 5 where the recess 7 is not formed.
Further, there are variations in size and characteristic of each component of the motor including the magnetic sensor such as a Hall element, the rotor yoke 5, and the rotating magnet 6. There also occur variations in assembling of each component. In addition, there are variations in characteristic of the processing circuit. As a result, the combination of these variations causes a possibility that if a magnetic sensor having a good sensitivity is located close to the rotor yoke 5, the level at the point e may exceed the threshold Th. In this case, the processing circuit erroneously detects two index positions per revolution of the rotor. Conversely, if a magnetic sensor having a poor sensitivity is located apart from the rotor yoke 5, there is a possibility that the level at the point b may not exceed the threshold Th in the worst case, causing a problem that the index detection becomes impossible. As a method for solving these problems, it may be considered to set individual thresholds according to the variations in size of each component and the sensitivity of the magnetic sensor. However, this method requires adjustment of the individual thresholds in manufacturing the motor or equipment using the motor, causing a reduction in productivity of the equipment and an increase in cost.